1. Field of the Invention
The present invention relates to an optical system including a diffractive optical element, the optical system being suitable to optical apparatuses, such as a still camera, a video camera and an interchangeable lens.
2. Description of the Related Art
For such optical systems, reduction in size (total length) and weight of the whole optical system tends to increase various kinds of aberrations, especially longitudinal (axial) chromatic aberration and chromatic aberration of magnification, and thereby to degrade optical performance of the optical system. For a telephoto optical system having a shortened total length in particular, a longer focal length leads to increased chromatic aberration. A known technique to reduce such chromatic aberration is to use a diffractive optical element disposed in the optical system. This technique can achieve a high achromatic effect, which cannot be achieved with a normal optical glass material, by utilizing a negative dispersion property and a high anomalous partial dispersion property of the diffractive optical element, which are different from the properties of normal glass materials.
The shortened total length of the optical system potentially results in increase in aberrations other than chromatic aberration. For example, an optical system of a typical telephoto lens includes, in order from an object side to an image side, a front lens unit having a positive refractive power, an aperture stop and a rear lens unit having a negative refractive power. For such an optical system, reduction in the total length of the optical system tends to require a larger power of the front lens unit, which results in a positive Petzval sum. This causes a problem that an image surface of the optical system excessively tilts to an under side.
Japanese Patent Laid-open No. 06-324262 discloses an optical system of a telephoto lens having a shortened total length by using the diffractive optical element. This optical system includes a diffractive optical element to correct, among chromatic aberrations increased with a shortened entire length of the optical system, a chromatic aberration for a g-line that cannot be corrected only with a normal achromatic condition specifying use of a low dispersive optical material and a highly dispersive optical material. The diffractive optical element has a high anomalous partial dispersion (θgF=0.296) as described above, so that it is effective in correcting the chromatic aberration for the g-line. Also the diffractive optical element has an absolute value of a value corresponding to an Abbe number as small as 3.453 so that it can correct chromatic aberration with a small refractive power caused by diffraction and little influence on spherical aberration, coma, astigmatism and the like.
To sufficiently correct chromatic aberration over the entire visible wavelength range, chromatic aberrations for wavelengths of d-, g-, C- and F-lines need to be removed. However, the configuration including the diffractive optical element disclosed in Japanese Patent Laid-open No. 06-324262 allows a relatively sufficient correction of the chromatic aberrations for three wavelengths among the d-, g-, C- and F-lines, but has difficulties in simultaneously correcting the chromatic aberration for the g-line and chromatic aberration for a wavelength range from the F-line to the C-line (especially the C-line). This is because the diffractive optical element causes chromatic aberration varying proportionally with wavelength. The chromatic aberration caused by the diffractive optical element and the wavelength are proportionally related to each other, and a gradient of the chromatic aberration for the wavelength range from the F-line to the C-line is the same as that of the chromatic aberration for a wavelength range from the g-line to the F-line.
In contrast, chromatic aberration caused in a typical optical material varies curvilinearly with wavelength, with a tendency of a larger curvature gradient for a shorter wave length. Therefore, the gradient of the chromatic aberration for the wavelength range from the F-line and the C-line differs from that of the chromatic aberration for the wavelength range from the g-line to the F-line. Thus, when the chromatic aberration caused in the typical optical material is corrected with a diffractive optical element, giving a refractive power to the diffractive optical element to correct the chromatic aberration for the wavelength range from the g-line to the F-line leaves the chromatic aberration for the wavelength range from the F-line to the C-line uncorrected. In an opposite manner, giving a refractive power to the diffractive optical element to correct the chromatic aberration for the wavelength range from the F-line to the C-line leaves the chromatic aberration for the wavelength range from the g-line to the F-line uncorrected. As described above, the use of the diffractive optical element has difficulties in simultaneously and sufficiently correcting the chromatic aberration for the g-line and the chromatic aberration for the C-line.
In addition, Japanese Patent Laid-open No. 06-324262 only discloses chromatic aberration as aberration increased with a shortened entire optical system length and a method of correcting the chromatic aberration. However, consideration should be made on increase in aberrations other than the chromatic aberration, especially increase in field curvature (excessive tilt of the image surface to the under side).